In the past, reciprocating piston pumps have been driven by reciprocating air motors. These air motors use compressed air to reciprocate a motor piston, which in turn is connected to a suitable reciprocating pump. Systems of this type delivered fluid through the pump at an irregular rate, caused principally by the non-linearities which occur at the time the air motor piston changes direction. Because of the compressibility of the air used to activate such systems, the air motor piston reacts slowly at its extreme displacement positions, commonly referred to as the "change-over." The finite time required for the compressed air supply to regenerate sufficient internal pressure to move the air motor piston causes a temporary reduction in pumped fluid flow, and thus the fluid flow is irregular during changeover.
The use of hydraulic motors has improved the uniformity of pumped fluid flow, because hydraulic motors utilize incompressible hydraulic fluid for their driving source and thus are able to go through changeover faster than air motors. In addition, the valving of hydraulic fluid is more positively controllable than air, and hydraulic motors therefore respond more directly to control valving. Thus, the use of hydraulic motors has improved the speed of changeover and thereby provided a more uniform output flow from the reciprocating pump connected to the hydraulic motor. However, this improved changeover speed causes additional problems which affect the metering and flow rate of the pumped fluid. An effect called "diving" becomes more pronounced, particularly when hydraulic motors are used to pump highly viscous materials and fluids.
Experimentation has shown that "diving" is caused by at least three factors. The first of these is fluid cavitation caused by the limited area of the pump inlet valve, which results in a significant pressure drop across the valve and thereby produces cavitation chambers in the pump cylinder below the pump piston. As the hydraulic motor goes through top changeover the force on the pump piston is suddenly reversed and a rapid downward piston movement occurs to fill the cavitation chambers. This sudden downward movement creates a pump cylinder flow rate demand above the piston which cannot be met because of the limited orifice size of the valving feeding the cylinder in this region. Therefore, additional cavitation chambers are produced in the pump cylinder above the piston to create the second factor causing "diving."
The third factor which causes "diving" is valve closure loss which usually occurs after the first mentioned cavitation chambers have been filled but before the second mentioned cavitation chambers have been filled. The pump piston, in its downward movement, fills the first mentioned cavitation chambers but continues to move downward rapidly because the pump inlet check valve remains open for a finite time. Pumped fluid flowing around this check valve adds a downward drag force to assist in closing the valve, but that same material is lost until the valve is fully closed. Once the inlet check valve is closed, pump piston velocity remains above its steady state value until the second mentioned cavitation chambers have been filled. After this occurs, the pump reaches a steady state velocity and thereafter operates as a positive displacement pump. However, during the "diving" period, the pump does not operate as a positive displacement pump, delivering less pumped fluid per increment of displacement than it delivers during steady state operation. Therefore, "diving" not only produces poor fluid flow but contributes to accuracy errors in metering the amount of fluid pumped.
The present invention overcomes the disadvantages caused by "diving" by providing an improved hydraulic valving mechanism which overcomes the cavitation problems that cause "diving."